This invention is an improvement to the high speed melt spinning of synthetic polymer fibers. Via this invention, the structure and properties of the as-spun fibers such as orientation, density, crystallinity and tensile properties are significantly improved for spinning in the high speed range. This approach may be applicable to the melt spinning process of several different synthetic polymers. It is expected that the orientation and crystallinity of any melt spinnable polymers with relatively low crystallization rates can be increased by this approach.
Many factors influence the development of threadline orientation and crystallinity in the conventional melt spinning process, in which molten filaments are extruded from spinneret holes and are usually rapidly cooled to room temperature by a cross-flow air quench. The fibers so produced normally possess low orientation and crystallinity at low take-up speeds. Since orientation of the as-spun fibers increases almost linearly with increasing take-up speed, take-up speed has historically been the most effective parameter in controlling the structure development in the threadline. Medium speeds between 2500-4500 m/min yield partially oriented yarns (POY) which, due to low crystallinity, have too much elongation potential and creep, or non-removable potential elongation, for use in most textile applications. Characteristically, however, significant crystallization starts to develop in the threadline as take-up speeds exceed 4500 m/min, producing more fully oriented fibers.
An ideal industrial process for synthetic fiber spinning should be simple and effective and should yield fibers having a high degree of orientation and crystallinity. Most commercial synthetic fibers are presently manufactured by a coupled two-step process (TSP): (i) spinning at low speeds of approximately 1000-1500 m/min to produce fibers having a relatively low degree of orientation and crystallinity; and (ii) drawing and annealing under certain conditions to increase the orientation and crystallinity in the fibers. However, because of the crystallization characteristic of synthetic polymers, much academic and industrial research has in recent years focused on developing a one-step process (OSP) for high speed spinning. Numerous patents and publications concerning high speed spinning by many investigators have recently appeared, and the book High Speed Fiber Spinning gives a literature and patent survey of recent developments in high speed spinning. Ziabicki and Kawai, Eds., High Speed Fiber Spinning, Wiley Interscience, New York (1985).
Many technical problems have been encountered in adapting current production schemes in the course of developing an OSP for high speed spinning. For example, a speed limit exists at which fiber orientation, crystallinity, and many other properties are maximized, implying that take-up speed cannot be infinitely increased under existing spinning conditions. Frequent filament breakage, high skin-core differences in fiber structure and low amorphous orientation are also encountered at very high take-up speeds.
To avoid or minimize the above problems, several techniques have been developed for spinning fibers at high take-up speeds. A common practice is to delay the quench rate of the molten filament. Yasuda studied the effect on polyethylene terephthalate (PET) of varying cooling air temperature from 22.degree. C. to 98.degree. C. and found that the differential birefringence (.delta..DELTA.n) of PET decreased as cooling air temperature increased. High Speed Fiber Spinning at Ch. 13, p. 363. Frankfort placed a heated sleeve immediately below the spinneret to delay the quench rate U.S. Pat. No. 4,134,882. Use of a high length-to-diameter ratio (L/D) in the capillary die, a modification believed to raise the surface temperature of the extrudate, has also been reported to reduce .delta..DELTA.n.
Vassilatos et al. used hot air to slow the cooling rate of the entire spinline, in order to decrease excessive spinline breaks at speeds above 6400 m/min. High Speed Fiber Spinning at Ch. 14, p. 390. However, slowing the cooling rate with hot air or other means alone cannot lead to an increase in either birefringence or crystallinity, probably because the relaxation time of the polymer molecules decreases with increasing temperature. When the cooling of the molten filament is materially delayed by use of a heated sleeve or flow of hot air around the fiber, considerable deformation occurs in the relatively high temperature region and flow-induced orientation is readily relaxed. However, if the molten filament is initially cooled very rapidly, the temperature of the filament can be brought to an optimum temperature to effectively obtain a flow-induced orientation which can be retained without significant thermal relaxation. This characteristic is likely related to the increased relaxation time and theological stress of synthetic fibers due to their greater viscosity at low temperatures. The mechanism of structure formation in melt spun fibers is complex since it is not an isothermal process. The crystallization rate of a threadline depends upon both the temperature and the level of molecular orientation induced by melt flow in the threadline. Since flow-induced orientation is influenced by the development of the deformation, minimizing thermal relaxation while deforming the fiber rapidly at a relatively low temperature should achieve a high level of orientation. Under certain conditions, molecular orientation increases with increasing deformation rate, which is in turn proportional to take-up velocity. Increased flow-induced orientation therefore results in a high rate of crystallization and crystallinity in the fibers spun.
Many researchers have observed a necking phenomenon occurring in PET fibers during the high speed spinning process and report that the filament is essentially amorphous above the necking zone whereas crystallinity is either maximized or unchanged afterwards. Necking may therefore indicate the region of the maximum rate of crystallization in the threadline. Recent studies show the neck occurring in the threadline at a distance varying between 130 cm and 50 cm from the spinneret for speeds ranging from 4000 m/min to 7000 m/min, respectively, so that the neck moves closer to the spinneret as take-up speed increases. threadlines temperature at the neck also increases from 130.degree. C. to 180.degree. C. with increasing speed. George, Holt, and Buckley, Polym. Eng. & Sci., Vol. 23, 95 (1983). The crystallinity of the spun fiber and its level of crystal orientation can be increased or even maximized by maintaining the filament near optimum conditions for a relatively long time since final crystallinity is an integration of the crystallization rate and crystallization time.
Previous studies obtained ultra-oriented PET strands by using convergent die geometries to produce an elongational flow field. Ledbetter, Cuculo, and Tucker, J. Polym. Sci., Polym. Chem. Ed.Vol. 22, 1435 (1984), Ihm and Cuculo, J. Polym. Sci., Polym. Physics Ed., Vol. 25, 2331 (1987). Application of high pressure to the polymer flowing through the convergent die produced rapid crystallization which effectively locked in the molecular orientation induced by the elongational flow. The birefringence of the oriented strands, was between 0.196 and 0.20, which is higher than that of conventional, fully drawn yarn. The present invention extends that work from a batch process to a continuous one.